At the bottom of the world, on the edge of the Antarctic continent, surrounded by ice and ocean, a scene of all-out war is taking place. A Japanese fishing vessel is firing water cannons and pounding a smaller ship, the Sea Shepherd’s Bob Barker, repeatedly ramming it against the side of a tanker. The activist-populated boat is positioned strategically, preventing the whalers from refuelling.

In the vicinity other boats jostle for positions: another two Sea Shepherd ships and five other whaling vessels. The chaotic tableau strikes me as the kind of scene that only plays out in high-seas war movies. It sounds like a cliche, but watching it I have to remind myself to keep breathing.

The Bob Barker loses power. A life raft is smashed to smithereens. Captain Peter Hammarstedt, who now can’t move his ship even if he wants to, issues a mayday. Hammarstedt is kind of a reverse Ahab – with the unflappable resolve of the Moby Dick captain, but on a mission to save whales rather than kill them.

“I remember very clearly standing on the bridge of the Bob Barker as we’re in the midst of this almost naval-like battle,” he tells Guardian Australia.

“Eight ships in all, fighting over their positions, us trying to prevent this whaler from refuelling. The sight of what happened that day gives me hope. So many people were there, offering so much to make a difference for a species other than our own.”

This pulse-pounding moment was captured in the Australian documentary Defend, Conserve, Protect, which charts the Sea Shepherd Conservation Society’s ninth campaign, called Zero Tolerance, during which it policed the world’s only international whale sanctuary.

It is a rousing film, directed by Stephen Amis and constructed using footage captured from four Sea Shepherd vessels, which were fitted with camera and GoPros. Amis has described the film as one that “captures the spirit of global activism – where real people show raw, unvarnished fear and heroism against terrifying odds”.

Indeed, most war films I watch aren’t as intense as this. Activist documentaries of this ilk are inevitably preaching to the converted, even if that’s not their intention, as they’re likely to attract audiences who were enthusiastic champions of the cause before they walked in the door. And yet I find myself thinking I wouldn’t be surprised if viewers who aren’t gung-ho Sea Shepherd supporters left the film feeling some kind of respect for the people involved, given their unquestionable grit and tenacity.

Formally incorporated in 1981, after being formed as an idea in the late 1970s by Captain Paul Watson – who was also a co-founder of Greenpeace – the not-for-profit organisation uses direct action activism to pursue its objectives. These include ending the destruction and slaughter of oceanic wildlife.

“People call us violent,” Watson says. “But I call what we do aggressive non-violence.” This “aggressive non-violence” has, perhaps inevitably, attracted controversy over the years, though the veteran activist insists that their record “is impeccable”, having “never caused a single injury to a single person in 42 years of operation.”

Watson says the core question the organisation keeps coming back to is: “How do you compete in a world where governments and corporations have a monopoly on violence?” He says it’s a “difficult” question and it “takes a lot of thought, a lot of imagination and a lot of courage in order to work within that framework.”

Joining the Sea Shepherd is dangerous work. In addition to the risks posed by direct confrontations, such as the one featured in the film, the Antarctic is dangerous in and of itself, with some of the most extreme weather in the world. Hammarstedt says he has experienced “12m seas off the Antarctic coastline”.

This is one of the reasons why, Hammarstedt says, “The one question we always ask of any member who wants to join these campaigns is: are you willing to risk your life to save the life of a whale? Unless that answer is a resounding yes, there isn’t really a spot on the ship for them.”

“People think that’s a little extreme,” adds Watson, “but we ask young people all the time to risk their lives, and even to actually give their lives, for wars over real estate, oil and religion. I think it is a far nobler pursuit to take a risk to protect an endangered species or an endangered habitat.”

The veteran activist says one defining moment in his activism came in the 1970s, when he was a medic during the occupation of Wounded Knee by the American Indian Movement. There were “360 of us surrounded by 3,000 federal troops”. Watson turned to Russell Means, one of the leaders of the movement, and asked: “Why are we here, when we don’t have any chance of winning this battle?”

Watson says Means’ response has stayed with him ever since.

“He said we’re not here because we’re concerned about the odds against us. And we’re not here because we’re concerned about winning or losing. We’re here because this is the right place to be, the right thing to do, and the right time to do it. You do what you can in the present and that contributes to whatever the future is going to be. Ever since, that’s been my philosophy.”

• Defend, Conserve, Protect is in Australian cinemas now